This invention relates to the treatment of fluid flows to remove colloidal matter, and, more particularly, to a media filter that removes colloidal matter.
Fluid flows often contain solid and soluble contaminants that must be removed before discharge or reuse of the fluid. For example, an industrial operation may use water to wash product. The wash water can contain large solid particles, very small solid particles termed colloidal particles, and soluble species. Before the wash water can either be discharged to the sewer system or reused in the wash operation, both the particles and soluble species must be removed to acceptably low levels as required by environmental regulations or process flow requirements.
In one common cleaning approach, during on-line operation the contaminated fluid is passed through a media filter containing a packed, finely divided filter media material such as crushed coal, sand, and gravel. The large particles in the fluid flow are captured within the filter media, for later removal by off-line backflushing. The colloidal particles are not readily captured by the filter media, because they are so small that they pass through the passageways within the filter media. To permit capture of the colloidal particles, it is a common, but not universal, practice to add a small amount (typically about 2-20 parts per million) of a coagulant to the fluid flow, upstream of the media filter. The coagulant causes the colloidal particles to coalesce together into larger particles, which can be captured within the media filter.
After the solid matter is removed in the filter, the soluble species are removed from the fluid. In one approach, the fluid is passed across a reverse-osmosis (RO) membrane, in which salts and other soluble components are removed. The result is a clarified fluid flow having a large fraction of both solid and soluble impurities reduced to acceptably low levels. It is a conventional practice to add an anti-scalant compound to the fluid flow after it leaves the media filter and before it enters the reverse-osmosis unit. The anti-scalant compound inhibits the deposition and buildup of scale within the reverse-osmosis unit. The presence of such scale inhibits the operation of the reverse-osmosis unit.
Such systems work well in an idealized operation, where precisely the right amount of coagulant is added upstream of the media filter. However, in actual practice problems can arise if exactly the right amount of coagulant is not added. If too little coagulant is added, some of the colloidal matter is not captured and reaches the reverse-osmosis unit or the outflow. If too much coagulant is added, the coagulant reacts with the anti-scalant compound to form a thick, sludge-like residue that can foul the pipes and the reverse-osmosis membrane. Cleanup after fouling is a major operation requiring extensive downtime of the plant.
It is difficult to know precisely the amount of coagulant to add, because the water quality and fluid flow rate can vary over time. As a result of the risk of fouling due to the reaction of excess coagulant and anti-scalant compound, most fluid-treatment plants of this type operate with no coagulant addition at all, or a deficiency of coagulant addition as compared with the optimal value for removing all of the colloidal material. Reverse-osmosis membranes therefore are prone to more rapid fouling due to deposition of colloidal matter on the membranes, and the final process water quality is not as good as it might otherwise be if the colloidal matter were more completely removed.
There is a need for a better approach to the treatment of fluid flows such as process and municipal water flows, to remove both the large and colloidal particulate matter, as well as the soluble impurities. The present invention fulfills this need, and further provides related advantages.